LT3508 LINER [Linear Technology], LT3508 Datasheet - Page 9

• Manufacturer Part Number: LT3508
• Description: Dual Monolithic 1.4A Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

Setting the Output Voltage
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
R2 should be 20k or less to avoid bias current errors.
Reference designators refer to the Block Diagram.
Minimum Operating Voltage
The minimum operating voltage is determined either by
the LT3508’s undervoltage lockout or by its maximum duty
cycle. If V
lockout is at 3.7V or below. If the two inputs are used
separately, then V
or below and V
below. Because the internal supply runs off V
nel 2 will not operate unless V
is the fraction of time that the internal switch is on and is
determined by the input and output voltages:
Unlike many fi xed frequency regulators, the LT3508 can
extend its duty cycle by turning on for multiple cycles. The
LT3508 will not switch off at the end of each clock cycle if
there is suffi cient voltage across the boost capacitor (C3
in Figure 1). Eventually, the voltage on the boost capacitor
falls and requires refreshing. Circuitry detects this condi-
tion and forces the switch to turn off, allowing the inductor
current to charge up the boost capacitor. This places a
limitation on the maximum duty cycle as follows:
where β
BOOST pin current as shown in the Typical Performance
Characteristics section. This leads to a minimum input
voltage of:
APPLICATIONS INFORMATION
R
DC
DC
V
IN MIN
1
=
MAX
(
=
R
SW
V
2
IN1
IN
)
=
⎛
⎝ ⎜
V
is equal to the SW pin current divided by the
=
V
0 8
–
OUT
1
and V
OUT
.
V
+
V
OUT
IN2
DC
SW
V
β
1
+
IN1
SW
MAX
–
1
IN2
has an undervoltage lockout of 3V or
V
+
+
1
F
⎞
⎠ ⎟
has an undervoltage lockout of 3.7V
V
V
are tied together, the undervoltage
F
F
–
V
F
+
IN1
V
SW
> 3.7V. The duty cycle
IN1
, chan-
where V
(~0.4V) and V
(~0.4V at maximum load).
Example: I
β
Maximum Operating Voltage
The maximum operating voltage is determined by the
Absolute Maximum Ratings of the V
and by the minimum duty cycle:
where t
is equal to 150ns) and f is the switching frequency.
Running at a lower switching frequency allows a lower
minimum duty cycle. The maximum input voltage before
pulse skipping occurs depends on the output voltage and
the minimum duty cycle:
Example: f = 790kHz, V
= 0.103:
The LT3508 will regulate the output current at input voltages
greater than V
output voltage of 1.8V and switching frequency of 1.5MHz
has a V
operation at 18V. Output ripple and peak inductor current
have signifi cantly increased. Exceeding V
the output is in regulation, if the external components have
adequate ratings to handle the peak conditions and if the
peak inductor current does not exceed 3.2A. A saturating
inductor may further reduce performance. Do not exceed
V
greater than 5V, use V
operation above 20V in pulse skipping mode, program
the switching frequency to 1.1MHz or less.
SW
IN(PS)
DC
V
V
V
IN MIN
IN PS
IN PS
= 1.5A/50mA = 30, DC
MIN
(
(
(
IN(PS)
during start-up or overload conditions (for outputs
ON(MIN)
F
)
)
= t
is the forward voltage drop of the catch diode
)
=
=
SW
=
ON(MIN)
of 11.3V, as shown in Figure 2. Figure 3 shows
3 3
V
SW
3 3
OUT
.
IN(PS)
= 1.5A and I
DC
.
is equal to 130ns (for T
0 103
V
is the voltage drop of the internal switch
V
.
MIN
96
+
+
+
. For example, an application with an
0 4
%
• f
V
0 4
.
F
OUT
OUT
.
V
–
V
V
– .
= 3.3V, DC
= 5V to calculate V
– .
F
BOOST
MAX
0 4
+
0 4
V
V
= 1/(1+1/30) = 96%:
SW
V
+
= 50mA, V
+
0 4
MIN
0 4
.
IN
.
J
V
and BOOST pins,
V
= 130ns • 790kHz
> 125°C t
=
=
IN(PS)
36
LT3508
3 8
OUT
.
V
IN(PS)
V
is safe if
ON(MIN)
= 3.3V,
). For
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